The Long Term Evolution (LTE) specifications have been standardized to support Component Carrier (CC) bandwidths up to 20 MHz (which may be the maximal LTE Rel-8 carrier bandwidth). Accordingly, LTE operation with bandwidths wider than 20 MHz may be possible and may appear as a number of LTE carriers to an LTE terminal.
A straightforward way to provide such operation could be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple Component Carriers CCs (also referred to as carriers), where each CC has (or at least has the possibility to have) the same structure as a Rel-8 carrier. An example of Carrier Aggregation CA is illustrated in FIG. 1.
The LTE standard may support up to 5 aggregated carriers where each carrier is limited in the Radio Frequency RF specifications to have one of six bandwidths, i.e., 6, 15, 25, 50, 75, or 100 Resource Blocks RB (corresponding to 1.4, 3, 5, 10, 15, and 20 MHz respectively).
The number of aggregated Component Carriers CCs as well as the bandwidth of each individual CC may be different for uplink and downlink (generically referred to as wireless communication links, communication links, or simply links). A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the numbers of CCs in downlink and uplink are different. A number of CCs configured in the network may be different from a number of CCs seen by a terminal. A terminal may, for example, support and/or be configured with more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
During initial access, an LTE CA-capable terminal may behave in a manner similar to a terminal not capable of CA. Upon successful connection to the network, a terminal may (depending on its own capabilities and the network) be configured with additional CCs in the UL and DL. Configuration may be based on Radio Resource Control RRC. Due to the heavy signaling and rather slow speed of RRC signaling, it is envisioned that a terminal may be configured with multiple CCs even though not all of them are currently used. If a terminal is activated on multiple CCs, this would imply that it has to monitor all DownLink DL CCs for PDCCH (Physical DownLink Control CHannel) and PDSCH (Physical DownLink Shared CHannel). This operation may require a wider receiver bandwidth, a higher sampling rate, etc, resulting in increased power consumption.
In CA, the terminal is configured with a primary Component Carrier CC (or cell or Serving cell), which is referred to as the Primary Cell or PCell. The PCell may be particularly important, for example, because control signaling may be signaled on this cell and/or because the UE may perform monitoring of the radio quality on the PCell. A CA capable terminal can, as explained above, also be configured with additional component carriers (or cells or serving cells) which are referred to as Secondary Cells (SCells).
The terms terminal, wireless terminal, UE (User Equipment), and User Equipment node will be used interchangeably throughout this document.
In LTE, the eNodeB (also referred to as a base station) and the UE use Medium Access (MAC) Control Elements (CE) to exchange information such as buffer status reports, power headroom reports, etc. A comprehensive list of MAC CEs is provided in section 6.1.3 of 3GPP TS 36.321 v12.3.0 (2014-09), “LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification.” Moreover, each MAC CE may be identified by a LCID (Logical Channel Identity) which is used as an identifier for the MAC CE so that the receiver interprets the MAC CE correctly. With the existing LTE specification, however, a number of component carriers may be limited.